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Nutrient Sources and Gulf of Mexico Hypoxia

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USGS hydrologist Richard Alexander talks about the nine States that contribute to the majority of nutrients in the northern Gulf of Mexico, threatening the economic and ecological health of one of the Nation's largest and most productive fisheries.




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Music credit:

"Flight of the Bumblebee" by Nikolai Rimsky-Korsakov



Jennifer: Welcome, and thanks for listening to the USGS CoreCast. I'm Jennifer LaVista.

The USGS has identified the major agricultural practices in nine states that contribute to the majority of nutrients in the Northern Gulf of Mexico. Today we are joined by USGS hydrologist, Richard Alexander. Richard will tell us about the findings of the study, why nutrients are of such a concern and what can be done to address these nutrient problems in the Gulf. Richard, thanks for joining us

Richard: It's great to be here

Jennifer: Now first off, tell me about some of the major findings in the study.

Richard: Sure, Jenn. Agricultural practices are what we found are the principal sources of nitrogen and phosphorus in the Gulf. In fact, they contribute about 70 percent of both the nitrogen and phosphorus. We found that urban sources on the other hand contribute only about 10 percent of those nutrients. With regard to the agricultural sources, we found that much of the nitrogen comes from corn and soybeans. There's a very large corn belt within the Mississippi River Basin where about 80 percent of the corn in the United States is actually grown. Nitrogen tends to be fairly mobile in the environment because it's in a dissolved form. So it readily runs off from these types of lands and inner streams.

By contrast to this though, when we looked at phosphorus what we found is that animals in pasture and range really matter quite a bit within the Mississippi River Basin. We found that phosphorus in the animal manure is nearly as important a contributor as cultivated row crops. By comparison, we found that about 37 percent of the phosphorus that enters the Gulf comes from pasture and rangeland and animal manure on those lands. Compared to about a little more than forty percent from the row crops. These contributions of phosphorus from animal manure is larger than previously recognized and they suggest that perhaps more management attention to animals on pasture land may be needed.

Jennifer: So what might explain that difference?

Richard: Well, some of the factors that may explain it include phosphorus enrichment in the waste itself. Animals don't absorb the phosphorus as readily as nitrogen, so that may lead to greater phosphorus in the rivers and streams, also animals compact the soil when they tread around on the soils and paddocks and animal wading in streams may introduce waste directly in the streams. By contrast, on row crops there's a lot of use of conservation tillage, which may hold up the phosphorus and not allow it to run off as readily, which may raise the importance of pasture lands as a source within the basin.

Jennifer: Okay, so where in the Mississippi River Basin are these nutrients coming from?

Richard: Well, most of the nitrogen and phosphorus come from the central parts and eastern parts of the Mississippi Basin. These are areas where a lot of the agricultural land is located, there are also large cities that are located on the major rivers, such as the Ohio and the upper parts of the Mississippi, and there are a lot of nutrients that come from the wastewater treatment plants from those cities. One of the interesting findings is that only about a third of the drainage area contributes more than about three quarters of the nitrogen and phosphorus to the Gulf. And this area, consisting of about a third of the drainage covers a nine state area throughout the Mississippi River Basin.

Jennifer: And what are those states?

Richard:Well, it includes Iowa, Illinois, Indiana, Missouri, Arkansas, Kentucky, as well as Tennessee, Ohio and Mississippi are all in this major contributing region of the Mississippi.

Jennifer: Are there any other factors besides land use that contribute to nutrients in the Gulf?

Richard: Well, there is an interesting factor other than land use, and that is the river network itself. It's very important, the streams, rivers, the reservoirs all have an important impact on nutrient delivery in the Gulf. These central and eastern parts of the basins that contribute so much of the nitrogen and phosphorus are in fact drained by very large rivers.

These large rivers are where very little nitrogen and phosphorus are naturally removed. And so this helps explain why the nutrients are delivered from really far upstream within the basin over thousands of miles nutrients can be transported with relatively little loss. The rivers also connect to smaller streams and in some cases are linked to tile drain systems, which are put in underneath corn and soybean and other types of crops. And so that helps to connect the nutrients where they are applied to the land and helps connect them to further downstream where they ultimately end up in terms of the Gulf.

Jennifer: Now tell me about why these nutrients are such a concern in the Gulf?

Richard: Well the nutrients we are talking about here, the nitrogen and phosphorus which we mentioned before are essential elements which are required for the organisms to really live. The entire food web, from very large fish to small organisms all require nitrogen and phosphorus to survive. But, too much of a good thing can lead to a problem in the Gulf of Mexico. An overabundance of these elements can lead to hypoxia.

Jennifer: So, what is hypoxia?

Richard: Hypoxia, basically is a condition of low dissolved oxygen in the water. All of these organisms that live in the Gulf require, just like we do, oxygen in order to sustain themselves. The problem is when you have an overabundance of nutrients, you can get excessive growth of these algae in the Gulf. And, what happens when they go through their normal life cycle and the algae die, bacteria feed on the dead algal cells, which requires oxygen, and so that can strip the oxygen out of the water.

One of the concerns here is that this can cause quite a bit of stress on the ecosystem, in particular the bottom dwelling organisms because that is where the oxygen is typically used up first. And the other issue is that poses a threat to economic and ecological health of the estuary. In fact, the Gulf is one of the nation's most productive fisheries.

Jennifer: So, how big is the hypoxic zone in the Gulf?

Richard: Well, in 2007 the hypoxic zone was about 8,000 square kilometers, and that's about the size of the state of Massachusetts.

Jennifer: Wow

Richard: That's the third largest hypoxic area since 1985, and that's when scientists first began taking measurements. Since then, hypoxia occurs each year, typically during the summer months, the June, July and August time frame, sometime in that period. And, it's shown fairly large increases since 1985 and also just fluctuations, large fluctuations from year to year.

Jennifer: Now, does this occur anywhere else in the U.S. or around the globe?

Richard: Yes, that's an interesting question. Actually hypoxia has been occurring around the world and a number of estuaries over the last several decades, similar to what we've observed in the Gulf of Mexico. These are typically shallow estuaries, typically have very large river inflows to these that are dominated often by fresh water inflows. And this often leads to setting up the conditions that are required to create hypoxic conditions. Incidentally, both the Gulf and globally there have been large increases in the river nutrients that flow into these esturaries.

Jennifer: And, why is that?

Richard: Well, what's happened over the past several decades is that population, obviously, has increased in the U.S. and globally quite a bit. And this population, of course, requires food in order to survive and so there's been a particular increase in the use of farm fertilizers to sustain that population. At the same time there has also been many more vehicles on the road, power plant emissions, all of those add nitrogen to the atmosphere, which is deposited into watersheds and can run off into rivers. So that's another source of nitrogen that's considered to be important as well.

Jennifer: So tell me a little bit about how this study was conducted.

Richard: Sure. I can give you a few details about the model and how we did the study. We used a watershed model called SPARROW to describe how the water and the nutrients travel through the watershed to the Gulf from different types of land uses throughout the basin. The model describes the landscape in half square miles pieces, if you can imagine that. These are all located in relation to the nearest receiving river or reservoir, which are all connected together in a river network that consists of about 600,000 miles of streams within the United States.

Jennifer: So that covers a lot of ground!

Richard: It does. It's a very big model and it makes use of a lot of geospacial information that is collected by the federal government and other entities in the U.S. What we use the model to provide is basic geographic framework for really understanding water flow and nutrient movement. But, really to obtain reliable estimates of nutrient delivery to the Gulf and also information about the nutrient sources, the model depends pretty heavily upon having this detailed spacial measurements and data that are referred to. This includes a variety of sources - one of them, includes USGS measurements of nutrients and stream flow of rivers. These are taken at selected locations on rivers within the basin and over long periods of time.

We also use in the model, and combine with these USGS measurements a lot of geospacial data on land use, which is based on satellite measurements. And so, it's an incredible set of individual pieces of data that when you bring them together under a watershed model, they really provide interesting insight into where the nutrients originate from, and ultimately their fate - where they end up, such as the Gulf of Mexico.

Jennifer: Now that we have these findings, what can be done about this problem?

Richard: Well, one of the things that's under consideration is setting more stringent nutrient reduction goals for the Mississippi River Basin. The Gulf of Mexico Hypoxia Task Force has the lead for setting these reduction goals. This is a federal and state effort to better manage nutrients in the Basin. The Task Force is currently considering reduction goals for both nitrogen and phosphorus of 45 percent, which they'd like to try to achieve by 2015. This is part of an effort to try to reduce the size of the hypoxic zone by half, also by 2015.

Jennifer: Were any reduction goals previously set for nitrogen and phosphorus?

Richard: Well, a reduction goal was set for nitrogen by the Task Force in 2001. They set a goal of reducing nitrogen by 30 percent by 2015. However, this is the first time that a reduction goal is being considered for phosphorus. So, one of the useful pieces of information from our study is the new focus on phosphorus. The study provides useful information to the Task Force and the States about the important sources of phosphorus in the basin.

Studies like this also are helpful to scientists and researchers to really help define what the next steps are, they help to identify what the data gaps are, some exist currently in monitoring information for example. Our networks have declined over the past couple of decades. And it can also help to identify other important pieces of information, like the agricultural sources and the best management practices that are being used within the watershed.

Jennifer: Well great. Anything else you'd like to add?

Richard: That's all.

Jennifer: Well thank you so much for joining us Richard, it's been really great.

Richard: Thank you very much.

Jennifer: And thanks to all of you for listening to this episode of CoreCast. You can find out more information by logging onto CoreCast is a product of the U.S. Geological Survey, Department of the Interior. Until next time, I'm Jennifer LaVista.


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Music credit:

"130 GuitarOverdrive2 ", dimlif

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